Isolated parallel ups system with fault location detection

ABSTRACT

An uninterruptible power supply (UPS) system is provided. The UPS system includes a plurality of UPSs, a ring bus that electrically couples the UPSs together, a static switch coupled between an associated UPS of the UPSs and the ring bus, and a controller. The controller receives current data representative of an inverter current and a load current associated with the associated UPS. An output capacitor of the associated UPS is coupled to a node that conducts the inverter current and the load current. The controller further calculates a measured current based on the received current data, determines a voltage of said output capacitor, generates a derived current based on the determined voltage and a predetermined capacitance of said output capacitor, compares the measured current and the derived current to identify a fault location, and controls said static switch based on the identified fault location.

BACKGROUND OF THE INVENTION

The field of the invention relates generally to uninterrupted powersupplies (UPS), and more specifically, isolated parallel ring buses forinterconnected UPS systems including choke bypass switches.

Robust power systems enable supplying power to one or more loads. Suchpower systems may include combinations of generation, transport,rectification, inversion and conversion of power to supply energy forelectronic, optical, mechanical, and/or nuclear applications and loads.When implementing power systems and architectures, practicalconsiderations include cost, size, reliability, and ease ofimplementation.

In at least some known power systems, one or more uninterruptible powersupplies (UPSs) facilitate supplying power to a load. UPSs facilitateensuring that power is continuously supplied to one or more criticalloads, even when one or more components of a power system fail.Accordingly, UPSs provide a redundant power source. UPSs may be utilizedin a number of applications (e.g., utility substations, industrialplants, marine systems, high security systems, hospitals, datacomm andtelecomm centers, semiconductor manufacturing sites, nuclear powerplants, etc.). Further, UPSs may be utilized in high, medium, or lowpower applications. For example, UPSs may be used in relatively smallpower systems (e.g., entertainment or consumer systems) or microsystems(e.g., a chip-based system).

However, if a UPS fails or is malfunctioning, the load may not receivesufficient quality power to operate. In at least some known systems,multiple UPSs are coupled to a load to provide additional powerredundancy. If one UPS fails, the other UPSs provide power to the load.In these known systems, the transient caused by a UPS failure can reducethe power quality of the power supplied to the load. For example, insystems with a power distribution unit (PDU) isolation transformercoupled between the UPSs and the load, isolating a fault from a failedUPS may saturate the PDU isolation transformer, which affects the powerquality of the power supplied to the load.

BRIEF DESCRIPTION

In one aspect, an uninterruptible power supply (UPS) system is provided.The UPS system includes a plurality of UPSs, a ring bus thatelectrically couples the UPSs together, a static switch coupled betweenan associated UPS of the UPSs and the ring bus, and a controller. Thecontroller receives current data representative of an inverter currentand a load current associated with the associated UPS. An outputcapacitor of the associated UPS is coupled to a node that conducts theinverter current and the load current. The controller further calculatesa measured current based on the received current data, determines avoltage of said output capacitor, generates a derived current based onthe determined voltage and a predetermined capacitance of said outputcapacitor, compares the measured current and the derived current toidentify a fault location, and controls said static switch based on theidentified fault location.

In another aspect, a controller for identifying a fault location in aUPS system including a plurality of UPSs and a ring bus thatelectrically couples the plurality of UPSs together is provide. Thecontroller is communicatively coupled to a static switch coupled betweenan associated UPS of the UPSs and the ring bus. The controller receivescurrent data representative of an inverter current and a load currentassociated with the associated UPS. An output capacitor of theassociated UPS is coupled to a node that conducts the inverter currentand the load current. The controller further calculates a measuredcurrent based on the received current data, determines a voltage of theoutput capacitor, generates a derived current based on the determinedvoltage and a predetermined capacitance of the output capacitor,compares the measured current and the derived current to identify afault location, and controls the static switch based on the identifiedfault location.

In yet another aspect, a method for identifying a fault location withina UPS system that includes a plurality of UPSs and a ring bus thatelectrically couples the UPSs together is provided. The method is atleast partially performed by a controller of the UPS system. The methodincludes receiving current data representative of an inverter currentand a load current associated with a first UPS of the UPSs. An outputcapacitor of the first UPS is coupled to a node that conducts theinverter current and the load current. The method further includescalculating a measured current based on the received current data,determining a voltage of the output capacitor, generating a derivedcurrent based on the determined voltage and a predetermined capacitanceof the output capacitor, comparing the measured current and the derivedcurrent to identify a fault location, and controlling a static switchassociated with the first UPS based on the identified fault location.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary schematic diagram of an isolated parallel ringbus system of uninterruptible power supplies (UPSs) for providing powerredundancy to a load.

FIG. 2 is an exemplary schematic diagram of the system shown in FIG. 1in communication with a controller.

FIG. 3 is an exemplary schematic diagram of an isolated parallel ringbus system similar to the system shown in FIG. 1 with a static switch.

FIG. 4 is an exemplary schematic diagram of the system shown in FIG. 3during a UPS fault condition.

FIG. 5 is an exemplary schematic diagram of the system during the UPSfault condition shown in FIG. 4 for a single power phase.

FIG. 6 is a simplified diagram of a first exemplary scenario for faultdetection that may be used by the system shown in FIG. 3.

FIG. 7 is a simplified diagram of a second exemplary scenario for faultdetection that may be used by the system shown in FIG. 3.

FIG. 8 is a simplified diagram of a third exemplary scenario for faultdetection that may be used by the system shown in FIG. 3.

FIG. 9 is a simplified diagram of a fourth exemplary scenario for faultdetection that may be used by the system shown in FIG. 3.

FIG. 10 is a flow diagram of an exemplary method for identifying a faultlocation that may be used with the system shown in FIG. 3.

DETAILED DESCRIPTION

Exemplary embodiments of an uninterruptible power supply (UPS) systemare described herein. A plurality of UPSs are arranged in a ring busconfiguration and configured to supply power to at least one load. TheUPSs are each coupled to the ring bus through a respective choke toisolate the UPSs from each other. At least one static switch module iscoupled between an associated UPS and the ring bus to enable power fromother UPSs to bypass the respective choke when a fault condition occursat the UPS. A controller is communicatively coupled to the UPSs tomonitor and otherwise control the UPSs.

FIG. 1 is a schematic diagram of an exemplary UPS system 100 forproviding redundant power to a load. In the exemplary embodiment, system100 includes a first UPS 102, a second UPS 104, a third UPS 106, afourth UPS 108, a first switchgear 110, a second switchgear 112, a thirdswitchgear 114, a fourth switchgear 116, and a ring bus 118. In otherembodiments, system 100 includes additional, fewer, or alternativecomponents, including those described elsewhere herein.

In the exemplary embodiment, first UPS 102 is coupled to firstswitchgear 110. Similarly, second UPS 104 is coupled to secondswitchgear 112, third UPS 106 is coupled to third switchgear 114, andfourth UPS 108 is coupled to fourth switchgear 116. Each UPS 102, 104,106, 108 is configured to generate a power output. In the exemplaryembodiment, UPSs 102, 104, 106, 108 are rated to generate 1000 kilowatts(kW) of power. In some embodiments, UPSs 102, 104, 106, 108 areconfigured to store power and convert the stored power for transmission.In one embodiment, system 100 further includes fuses (not shown inFIG. 1) coupled to UPSs 102, 104, 106, 108 that are configured toelectrically disconnect UPSs 102, 104, 106, 108 from system 100 when afault condition occurs.

Switchgears 110, 112, 114, 116 are configured to receive the poweroutputs from the respective UPSs 102, 104, 106, 108 and transmit theoutputs to ring bus 118 or loads 120, 122, 124, 126. In the exemplaryembodiment, each load is coupled to a pair of switchgears throughseparate electrical connections (i.e., a “double corded configuration”)to provide additional redundancy to each load. For example, load 120 iscoupled between switchgears 110 and 112 to receive power from first UPS102 and second UPS 104. Power received at load 120 from third and fourthUPSs 106, 108 is transmitted through ring bus 118 to switchgears 110,112. In at least some embodiments, a power distribution unit (PDU)transformer is coupled between loads 120, 122, 124, 126 and system 100.

In the exemplary embodiment, switchgears 110, 112, 114, 116 include aplurality of electrical switches 128 that are configured to selectivelyopen and close in response to a control signal (e.g., from a controller(not shown in FIG. 1)). Switches 128 may be, for example, circuitbreakers. Switches 128 are positioned at various nodes withinswitchgears 110, 112, 114, 116 to facilitate locating and isolatingfaults within system 100. Switchgears 110, 112, 114, 116 further includechokes 130, 132, 134, 136, respectively. Chokes 130, 132, 134, 136 arecoupled between UPSs 102, 104, 106, 108 and ring bus 118. Chokes 130,132, 134, 136 facilitate load sharing within system 100 throughfrequency droop, and to limit fault current in case of a fault occurringat ring bus 118.

Ring bus 118 is configured to couple each UPS 102, 104, 106, 108together such that the UPSs are configured to limit fault current and toprovide additional power redundancy in the event of a fault condition ata UPS. Ring bus 118 includes a plurality of ring bus switches 138. Inthe exemplary embodiment, ring bus 118 is divided into data halls 140.Each data hall 140 is associated with a pair of UPSs and a pair of dualcorded loads. For example, one data hall 140 is associated with UPSs102, 104 and loads 120, 122. In the exemplary embodiment, ring bus 118includes two data halls 140. In other embodiments, ring bus 118 includesa different number of data halls 140. In one embodiment, each data hall140 is housed within a switchgear enclosure.

During a transient period after a faulted UPS is disconnected fromsystem 100, power from ring bus 118 passes through an associated choke.The associated choke creates a voltage drop by blocking a portion of thepower provided by ring bus 314, which causes the power quality at thePDU transformers and the loads coupled to the faulted UPS to be reduced.The associated voltage distortion may also cause saturation of themagnetic core of the PDU transformer, further reducing the power qualityat the loads. Additionally, the choke may prevent sufficient currentfrom passing to a fuse of the faulted UPS. With a limited fault currentfrom ring bus 114, the fuse remains intact and the faulted UPS remainsconnected to system 100, which may cause a reduction in power quality atthe load.

FIG. 2 is a partial schematic view of system 100 (shown in FIG. 1). Morespecifically, FIG. 2 is a schematic view of first UPS 102, firstswitchgear 110, partial ring bus 118, and a controller 200.

In the exemplary embodiment, controller 200 is communicatively coupledto UPS 102. Controller 200 is also communicatively coupled to UPSs 104,106, 108 within system 100 (each shown in FIG. 1). In other embodiments,a plurality of controllers may be used. In some embodiments, controller200 is coupled to a substitute controller (not shown) that may be usedin the event that controller 200 fails.

In the exemplary embodiment, controller 200 is implemented by aprocessor 202 communicatively coupled to a memory device 204 forexecuting instructions. In some embodiments, executable instructions arestored in memory device 204. Alternatively, controller 200 may beimplemented using any circuitry that enables controller 200 to controloperation of UPS 102 as described herein. For example, in someembodiments, controller 200 may include a state machine that learns oris pre-programmed to determine information relevant to which loadsrequire power. For example, controller 200 dynamically determines whatpower resources will be needed and at what performance level andenvironmental conditions (e.g., temperature, humidity, time of day,etc.) those power resources will need to operate. Controller 200 mayperform dynamic monitoring to determine whether a given load issatisfied with the power delivered, and whether delivered power is freeof harmonics, transients, etc. In some embodiments, dynamic monitoringincludes tracking resource usage to determine how much current orvoltage should be delivered. Controller 200 may also monitor and/orcontrol rapidity (i.e., bandwidth) and inverter capability (e.g.,overload, reactive power, active power) to facilitate ensuringreliability of system 100 and minimizing performance degradation of UPSs102.

Controller 200 may also include a state machine scheduler configured toselectively activate and deactivate power resources, set voltage andcurrent levels, and/or take power saving actions (e.g., reducing currentdelivery). Controller 200 may also track characteristics (e.g., staticallocation of power) of system 100 to determine whether one or morecomponents of system 100 should be put on standby or whether powershould be diverted.

In the exemplary embodiment, controller 200 performs one or moreoperations described herein by programming processor 202. For example,processor 202 may be programmed by encoding an operation as one or moreexecutable instructions and by providing the executable instructions inmemory device 204. Processor 202 may include one or more processingunits (e.g., in a multi-core configuration). Further, processor 202 maybe implemented using one or more heterogeneous processor systems inwhich a main processor is present with secondary processors on a singlechip. As another illustrative example, processor 202 may be a symmetricmulti-processor system containing multiple processors of the same type.Further, processor 202 may be implemented using any suitableprogrammable circuit including one or more systems and microcontrollers,microprocessors, reduced instruction set circuits (RISC), applicationspecific integrated circuits (ASIC), programmable logic circuits, fieldprogrammable gate arrays (FPGA), and any other circuit capable ofexecuting the functions described herein. In the exemplary embodiment,processor 202 causes controller 200 to operate UPS 102, as describedherein.

Controller 200 is configured to transmit and receive data from UPS 102.For example, controller 200 is configured to transmit data to UPS 102indicating ring bus 118 is connected. In another example, controller 200receives data from UPS 102 indicating a fault condition has occurred ormaintenance is required. Controller 200 is also configured to transmitcontrol signals to system 100. For example, controller 200 is configuredto adjust the magnitude, frequency, and/or phase of the power outputgenerated by UPS 102. In one embodiment, controller 200 monitors eachUPS and adjusts the operation of each connected UPS to synchronize thepower outputs of the UPSs. Power losses caused by mismatched poweroutputs may be reduced by synchronizing the power outputs. Inembodiments with multiple controllers, the controllers are incommunication to coordinate the operation of each UPS. In anotherembodiment, UPS 102 may directly control magnitude and frequency of thegenerated output power based on its own measurements. In one embodiment,UPS 102 may employ frequency droop control based on output active powerand voltage magnitude droop based on output reactive power. Series choke120 facilitates load sharing between the UPS modules, and the employeddroop techniques facilitate isochronous operation of all UPSs connectedto ring bus 118.

Controller 200 is further configured to monitor the circuit withinswitchgear 110 to detect fault conditions and other abnormal conditionsof system 100. In one embodiment, controller 200 is communicativelycoupled to a contactor 206 within switchgear 110. In some embodiments,contactor 206 is replaced with a relay. When a current or voltagedifferential monitored by contactor 206 exceeds a predeterminedthreshold, controller 200 is configured to selectively open or close oneor more switches 128, switches 138, and/or contactor 206 to electricallydisconnect UPS 102 from system 100 and protect the loads.

FIG. 3 is a schematic view of an exemplary UPS system 300. System 300 issubstantially similar to system 100 (shown in FIG. 1) and, in theabsence of contrary representation, includes similar components. In theexemplary embodiment, system 300 includes a first UPS 302, a second UPS304, a third UPS 306, a first switchgear 308, a second switchgear 310, athird switchgear 312, and a ring bus 314. In other embodiments, system300 includes additional, fewer, or alternative components, includingthose described elsewhere herein. For example, system 300 may include afourth UPS (not shown in FIG. 3). System 300 further includes controller200 (shown in FIG. 2) that is communicatively coupled to UPSs 302, 304,306.

Each UPS 302, 304, 306 includes a power storage device 303, such as abattery or capacitor, an alternating current (AC) to direct current (DC)converter 305, and a DC-to-AC inverter 307. In other embodiments, UPSs302 304, 306 have a different configuration. Power storage device 303 isconfigured to store electrical energy and provide the stored energy tothe loads. In the exemplary embodiment, power storage device 303 iscoupled between converter 305 and inverter 307. AC-to-DC converter 305is coupled to an external power source (not shown), such as a utilitygrid, and is configured to convert AC power received from the externalpower source into DC power for power storage device 303. Inverter 307 isconfigured to receive DC power from power storage device 303 andAC-to-DC converter 305 and convert the power to an AC power output forsystem 300. In the exemplary embodiment, controller 200 is configured tocontrol the operation of converter 305 and/or inverter 307 (e.g.,adjusting switching frequencies, etc.) to adjust the operation of system300 and the power supplied to the loads.

Each UPS 302, 304, 306 is coupled to a pair of PDU transformers and apair of loads. In the exemplary embodiment, UPS 302 and UPS 304 are eachcoupled to a first PDU transformer 316, a first load 318, a second PDUtransformer 320, and a second load 322. That is, loads 318, 322 arecoupled to system 300 in a dual-corded configuration (i.e., two UPSs areseparately connected to each load to provide redundant power). UPS 306is coupled to a third PDU transformer 324, a third load 326, a fourthPDU transformer 328, and a fourth load 330. In the exemplary embodiment,loads 326, 330 are in a single-corded configuration (i.e., a singleconnection to system 300 to receive power). However, loads 326, 330 mayfurther be coupled to another UPS (not shown).

Switchgears 308, 310, 312 include chokes 332, 334, 336, respectively.Switchgears 308, 310, 312 further include circuit breakers 338 that areconfigured to isolate faults within system 300 by selectivelydisconnecting a portion of system 300. In some embodiments, circuitbreakers 338 are monitored and controlled by controller 200. For ringbus applications, chokes 332, 334, 336 are sized to sustain a boltedfault on ring bus 314 for a sufficient time to isolate the fault throughthe activation of the specific breakers 338 in system 300. Further, forsituations where a breaker 338 fails to open, additional time may bebuilt-in to determine and execute an alternative fault isolationstrategy. Accordingly, to facilitate increasing a duration of time whereinverter 307 of an associated UPS 302, 304, or 306 can sustain a boltedfault on ring bus 314, chokes 332, 334, 336 may be sized to operateinverter 307 in a linear mode under a short circuit on ring bus 314.

To prevent limited fault current and reduced power quality at the loadsduring the transient period after a fault is detected at a UPS, system300 includes static switch modules 340, 342, 344. Static switch modules340, 342, 344 are coupled between ring bus 318 and UPSs 302, 304, 306,respectively. Static switch modules 340, 342, 344 may include, but arenot limited to, thyristors and insulated gate bi-polar transistors(IGBTs). In other embodiments, static switch modules 340, 342, 344 arereplaced with contactors, static transfer switches, and/or otherrelatively fast switching devices. In the exemplary embodiment, eachstatic switch module 340, 342, 344 includes a pair of static switches.In other embodiments, a different number of static switch modules and/orstatic switches per module may be included. Although it is shown thatstatic switch modules 340, 342, 344 are outside of switchgears 308, 310,312, it is to be understood that static switch modules 340, 342, 344 maybe within switchgears 308, 310, 312 or UPSs 302, 304, 306.

Static switch modules 340, 342, 344 are configured to selectively bypasschokes 332, 334, 336. In particular, static switch modules 340, 342, 344are configured to selectively bypass chokes 332, 334, 336 in response toa detected fault condition at an associated UPS. During normal operationof system 300 (i.e., no fault conditions have occurred), static switchmodules 340, 342, 344 are open. Static switch modules 340, 342, 344 areclosed in response to a fault condition detected at an associated UPS.Static switch modules 340, 342, 344 are configured to provide a lowimpedance path between the faulted UPS and ring bus 318, therebyfacilitating sufficient fault current to disconnect the faulted UPS andmaintaining or improving power quality at the loads using the powerprovided through ring bus 318. In the exemplary embodiment, the faultcurrent (i.e., current delivered in response to a fault condition)provided by ring bus 318 through static switch modules 340, 342, 344 isconfigured to be sufficient to blow a fuse associated with the faultedUPS to disconnect the faulted UPS from system 300. Using static switchesenables relatively fast reaction times to a detected fault in comparisonto circuit breakers 338 and other switching devices. In one example, anexemplary circuit breaker closes in approximately 50 milliseconds (ms)while an exemplary static switch closes in approximately 6-7 ms.

In the exemplary embodiment, controller 200 is communicatively coupledto static switch modules 340, 342, 344 to selectively open and closeswitches 340, 342, 344. Controller 200 is configured to detect a faultcondition, determine which (if any) UPS is associated with the faultcondition, and close a corresponding static switch to facilitate faultcurrent bypassing a choke. An exemplary detection and control method isdescribed in detail further below.

In some embodiments, static switch modules 340, 342, and 344 may be usedin a different power system that includes parallel inverters coupledtogether with a choke. That is, the systems and methods described hereinare not limited to UPS systems or UPS systems with a ring bus. The UPSsystems are for illustrative purposes only and are not intended to limitthe systems and methods as described herein. In one example, a staticswitch module may be coupled to an inverter parallel to a choke suchthat the static switch module is configured to selectively bypass thechoke.

FIGS. 4 and 5 are exemplary schematic diagrams 400, 500 of an exemplaryUPS system, such system 300 (shown in FIG. 3), during a fault conditionat a UPS. In particular, diagram 400 is a one-wire diagram of thethree-phase system, and diagram 500 is a simplified circuit showing onlyone of the phases, with ring 410 modeled as a single power source. Inthe exemplary embodiment, diagram 400 includes a faulted UPS 402, a fuse404, a choke 406, a static switch module 408, a ring bus 410, a PDUtransformer 412, and a load 414. Ring bus 410 includes three UPSs 416with respective chokes 418. In the exemplary embodiment, UPSs 416 areassumed to be operating without any fault conditions. Diagram 500includes a faulted UPS 502, a fuse 504, a ring bus 510, a PDUtransformer 512, and a load 514. Similar to ring bus 410, ring bus 510includes multiple UPSs, represented as a single power source 516 and achoke 518. In other embodiments, the UPS systems in diagrams 400, 500may include fewer, additional, or alternative components, includingthose described elsewhere herein.

With respect to FIGS. 4 and 5, the UPSs are represented as AC powersources. A fault condition has occurred at faulted UPSs 402, 502. Thatis, UPSs 402, 502 generate substantially no power or power differentfrom the power generated during normal operation of UPSs 402, 502. Forexample, the power factor of the power may be reduced. In the exemplaryembodiment, fuses 404, 504 are configured to electrically disconnectUPSs 402, 502 from ring bus 410, 510 and loads 414, 514 when fuses 404,504 receive a current that exceeds a predetermined current threshold.When the fault condition results in substantially no power generated byUPSs 402, 502, ring buses 410, 510 are configured to provide a faultcurrent that exceeds the current threshold to fuses 404, 504. Fuses 404,504 are melted by the fault current to electrically disconnect faultedUPSs 402, 502.

With respect to FIG. 4, the fault current bypasses choke 406 throughstatic switch module 408. Static switch module 408 is configured toprovide substantially no voltage or current drop. Accordingly, since thefault current bypasses the choke through the static switch, neither thechoke nor the static switch are shown in FIG. 5.

PDU transformers 412, 512 are configured to distribute power to loads414, 514. In the exemplary embodiment, PDU transformer 412 is adelta-wye transformer. Although a single load is shown in FIGS. 4 and 5,it is to be understood that multiple loads may be coupled to PDUtransformers 412, 512. Power is provided to PDU transformer 412 fromring bus 410 through static switch module 408. Similarly, power isprovided to PDU transformer 512 from ring bus 510. In addition toproviding power to PDU transformer 412 without the voltage drop causedby choke 406, static switch module 408 provides a low impedance path forthe fault current towards faulted UPS 402. With respect to diagram 500,the fault current would be provided by power source 516. Upon failure ofUPS 502, the voltage at its output terminal collapses. The resultingpotential difference across choke 518 drives a relatively steep rise ofthe current over it to feed the fault at UPS 502. Once the fault iselectrically disconnected by melting fuse 504 using the fault current,the magnetic flux applied to choke 518 to generate the fault current isthen balanced by a reverse-applied flux that drives the current down. AsPDU transformer 512 is exposed to the resulting voltage, the balancedflux balance applied to choke 518 also yields a balanced flux on PDUtransformer 512. Therefore, with respect to diagram 400, static switchmodule 408 is configured to facilitate maintaining flux balance on PDUtransformer 412 in the event of an internal fault on UPS 402.

Unlike static switch module 408, static transfer switches (STS) that maybe used in UPS systems are likely to cause transformer saturation whentransferring between asynchronous or out-of-phase power sources. Inparticular, in an example embodiment in which an STS feeds a PDUtransformer that includes a primary source connected to the output of aUPS and a secondary source fed by a utility or ring bus. The two sourcesare likely to exhibit phase shift, and an out-of-phase transfer betweenthe two power sources to the PDU transformer would drive the transformerinto saturation, compromising power quality to one or more criticalloads. The transfer may be delayed to avoid saturation, but theresulting power quality may not meet the demand of the load.

FIGS. 6-9 are simplified diagrams illustrating fault scenarios for anexemplary fault detection method to distinguish between UPS faults and aload fault that may be used by system 300 (shown in FIG. 3). Asdescribed above, closing a static switch module associated with afaulted UPS causes the fault current from the ring bus to bypass a chokeand blow a fuse of the faulted UPS. However, in the event of a fault atthe load, closing the static switch module causes the ring bus to beconnected to the fault. Accordingly, system 300 is configured todetermine a location of a fault and react accordingly. Although FIGS.6-9 only illustrate four exemplary fault scenarios, the exemplary faultdetection method may also be used to detect faults in other additionalfault scenarios and determine a location of the faults.

With respect to FIGS. 3 and 6-9, controller 200 is configured to monitorelectrical data at each UPS 302, 304, 306 to detect and locate a fault.In the exemplary method shown in FIGS. 6-9, electrical data associatedwith inverter 307 is monitored. Inverter 307 includes two parallelconverter modules 602 that generate an AC power output, an outputinductance 604, and an output capacitor 606. The AC power output istransmitted to an associated switchgear 308, 310, or 312 to be deliveredto the load and ring bus. In other embodiments, inverter 307 includes adifferent number of converter modules 602 (including one). Multiplemodules are driven by the same Pulse Width Modulation (PWM) signalgenerated by controller 200 based on voltage and current readings fromonly one of the converter modules 602.

Controller 200 is configured to collect measured current data associatedwith one output capacitor 606 and calculate a derived current associatedwith the same output capacitor 606. Based on the comparison of themeasured current data and the derived current, controller 200 isconfigured to determine a fault location (if any) and performappropriate response actions to isolate the fault. Controller 200 opensand closes static switch modules, circuit breakers, switches, and thelike within system 300 to isolate the fault. For example, if a UPS has afault condition, controller 200 is configured to close a respectivestatic switch to facilitate transmitting sufficient fault current fromthe ring bus to the fuse to disconnect the fuse. In another example, ifthe load has a fault condition, controller 200 is configured to open acircuit breaker between the faulted load and the UPSs of system 300 todisconnect the faulted line from the system.

Controller 200 is configured to monitor an inverter bridge currentI_(S), a load current I_(Load), and an output capacitor voltage V_(C)using one or more sensors (not shown). The sensors may be any type ofsensor that is configured to collect, calculate, or otherwise derivecurrent and/or voltage data. The data is collected periodically,continuously, and/or in response to a signal (e.g., a sensor alert, auser command, etc.). Controller 200 is configured to calculate ameasured current I_(C) as the difference between the inverter bridgecurrent I_(S) and the load current I_(Load). Controller 200 is furtherconfigured to calculate a derived current I_(D) using the outputcapacitor voltage V_(C) and a predetermined capacitance C of themeasured output capacitor 606 (I_(D)=C*dV_(C)/dt). In one embodiment,the predetermined capacitance C is a nominal or rated value of outputcapacitor 606. Controller 200 compares the measured current I_(C) andthe derived current I_(D). If the difference between current valuesexceeds a predetermined threshold, then a failure within the UPS iscausing at least a portion of the current from reaching the load. Usingdata collected from inverter 307 enables controller 200 to distinguishbetween faults at the UPS and faults at the load because a fault at theload does not cause the difference in the current values to exceed thethreshold. As such, controller 200 is configured to control system 300to isolate the fault based on the location of the fault.

For example, FIG. 6 is an exemplary diagram 600 of a first scenario inwhich the measured output capacitor 606 has shorted. The inverter bridgecurrent I_(S), and the load current I_(Load) are drawn to ground orneutral through the faulted output capacitor 606. The measured currentI_(C) is relatively greater than the derived current I_(D) such that thedifference exceeds the predetermined threshold. Accordingly, controller200 identifies the fault and determines that the fault is located at theUPS. In response, controller 200 closes the static switch to isolate thefault from system 300. Similar to FIG. 6, FIG. 7 is an exemplary diagram700 of a second scenario in which an unmeasured output capacitor 606 hasshorted. In the illustrated embodiment, the unmeasured output capacitor606 is the upper output capacitor 606. The measured current I_(C) isrelatively greater than the derived current I_(D) similar to the firstscenario and therefore controller 200 detects the fault.

FIG. 8 is an exemplary diagram 800 of a third scenario in which ameasured converter 602 has a failure while FIG. 9 is an exemplarydiagram 900 of a fourth scenario in which an unmeasured converter 602has a failure. In the third and fourth scenarios, a link capacitor (notshown) within the failed converter 602 collapses to substantially zeroimpedance. In the fourth scenario, similar to the first and secondscenarios, the difference between the measured current I_(C) and thederived current I_(D) exceeds the predetermined threshold and controller200 determines a fault at the UPS has occurred. However, in the thirdscenario, the measured current I_(C) and the derived current I_(D) areconsistent with each other, and the location of the fault remainsundetermined. In the exemplary embodiment, controller 200 iscommunicatively coupled to inverter 307, and inverter 307 is configuredto alert controller 200 when a link capacitor has collapsed. Forexample, in an exemplary embodiment, inverter 307 may be implemented asa Voltage Source Converter (VSC) with an associated DC-side capacitance.A fault on the DC-side capacitance (i.e., causing the impedance of theDC-side to collapse to substantially zero) causes a DC voltageassociated with the DC-side capacitance to collapse, thereby enablingprompt fault detection by detecting the drop in the DC voltage. Aconverter fault driving a short-circuit on the DC-side yields the sameeffects and triggers the same detection.

FIG. 10 is a flow diagram of an exemplary method 1000 for use with a UPSsystem, such as system 300 (shown in FIG. 3). In the exemplaryembodiment, method 1000 is at least partially performed by a controller(e.g., controller 200, shown in FIG. 3). In other embodiments, method1000 includes additional, fewer, or alternative steps, including thosedescribed elsewhere herein.

In some embodiments, method 1000 is performed continuously. In otherembodiments, method 1000 is performed periodically and/or in response toa control signal, such as a user command or a sensor alert signal. Thecontroller receives 1002 current data representative of an invertercurrent (e.g., inverter bridge current I_(S), shown in FIGS. 6-9) and aload current (e.g., load current I_(Load), shown in FIGS. 6-9) for oneor more UPSs in a ring bus configuration. An output capacitor is coupledto a node between the inverter current and the load current. Thecontroller further calculates 1004 a measured current based on thereceived current data. The controller determines 1006 a voltage of theoutput capacitor and generates 1008 a derived current based on thedetermined voltage and a predetermined capacitance (e.g., a nominalcapacitance) of the output capacitor.

The controller further compares 1010 the measured current and thederived current to identify a fault location and distinguish between aUPS fault condition and a load fault condition. Although referred to asa “load fault condition”, it is to be understood that a load faultcondition refers to a fault at any location external to the UPSs. In oneembodiment, the controller calculates a difference between the measuredand derived currents and compares the difference to a predeterminedthreshold. If the difference exceeds the threshold, the controllerdetermines a UPS fault condition has occurred. If the difference iswithin the threshold, the controller may determine a load faultcondition has occurred. In some embodiments, the controller may receivean alert signal from a UPS indicating a UPS fault condition has occurredirrespective of the comparison of the current difference and thethreshold. The controller controls 1012 a static switch based on theidentified fault location. For example, if the fault location is at theUPS associated with the static switch, the controller controls 1012 thestatic switch to close, thereby facilitating fault current from the ringbus to bypass a choke associated with the faulted UPS. In someembodiments, when a load fault condition is identified, the controllercontrols 1012 the static switch to remain open. In addition, thecontroller controls one or more circuit breakers or switches of the UPSsystem to isolate the location of the fault condition from the UPSs. Forexample, if a load has a fault condition, the controller causes one ormore circuit breakers between the UPSs and the load to disconnect.

The foregoing systems and methods are configured to facilitate improvedresponse times and improved power quality at a load in response to a UPSfault condition within a ring bus UPS configuration. The foregoingsystems and methods are further configured to facilitate improvedaccuracy and location of faults within a ring bus UPS configuration.

Exemplary embodiments of systems and methods for uninterruptible powersupplies are described above in detail. The systems and methods are notlimited to the specific embodiments described herein but, rather,components of the systems and/or operations of the methods may beutilized independently and separately from other components and/oroperations described herein. Further, the described components and/oroperations may also be defined in, or used in combination with, othersystems, methods, and/or devices, and are not limited to practice withonly the systems described herein.

The order of execution or performance of the operations in theembodiments of the invention illustrated and described herein is notessential, unless otherwise specified. That is, the operations may beperformed in any order, unless otherwise specified, and embodiments ofthe invention may include additional or fewer operations than thosedisclosed herein. For example, it is contemplated that executing orperforming a particular operation before, contemporaneously with, orafter another operation is within the scope of aspects of the invention.

Although specific features of various embodiments of the invention maybe shown in some drawings and not in others, this is for convenienceonly. In accordance with the principles of the invention, any feature ofa drawing may be referenced and/or claimed in combination with anyfeature of any other drawing.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. An uninterruptible power supply (UPS) systemcomprising: a plurality of UPSs; a ring bus that electrically couplessaid plurality of UPSs together; a static switch coupled between anassociated UPS of said plurality of UPSs and said ring bus; and acontroller operable to: receive current data representative of aninverter current and a load current associated with said associated UPS,wherein an output capacitor of said associated UPS is coupled to a nodethat conducts the inverter current and the load current; calculate ameasured current based on the received current data, wherein themeasured current is the difference between the inverter current and theload current; determine a voltage of said output capacitor; generate aderived current based on the determined voltage and a predeterminedcapacitance of said output capacitor; compare the measured current andthe derived current to identify a fault location; and control saidstatic switch based on the identified fault location.
 2. The UPS systemin accordance with claim 1, wherein said controller is further operableto: calculate a difference between the measured current and the derivedcurrent; compare the difference between the measured current and thederived current to a predetermined threshold; and determine the faultlocation is at said associated UPS when the difference between themeasured current and the derived current exceeds the predeterminedthreshold.
 3. The UPS system in accordance with claim 1 furthercomprising a circuit breaker, wherein said controller is furtheroperable to: calculate a difference between the measured current and thederived current; compare the difference between the measured current andthe derived current to a predetermined threshold; determine the faultlocation is at a location external to said associated UPS when thedifference between the measured current and the derived current iswithin the predetermined threshold; and control said circuit breaker toisolate the determined fault location.
 4. The UPS system in accordancewith claim 1, wherein said associated UPS comprises a plurality ofconverters, the inverter is current generated by a first converter ofsaid plurality of converters and the load current is an output currentof said plurality of converters.
 5. The UPS system in accordance withclaim 4, wherein said controller is further operable to: receive, fromsaid associated UPS, an alert signal indicating said first converter hasfailed to cause a UPS fault condition at said associated UPS; andcontrol said static switch to close in response to the alert signal. 6.The UPS system in accordance with claim 1, wherein said controller isfurther operable to control the static switch to close when thedetermined fault location is at the associated UPS.
 7. A controller foridentifying a fault location in an uninterruptible power supply (UPS)system including a plurality of UPSs and a ring bus that electricallycouples the plurality of UPSs together, said controller communicativelycoupled to a static switch coupled between an associated UPS of theplurality of UPSs and the ring bus, said controller operable to: receivecurrent data representative of an inverter current and a load currentassociated with the associated UPS, wherein an output capacitor of theassociated UPS is coupled to a node that conducts the inverter currentand the load current; calculate a measured current based on the receivedcurrent data, wherein the measured is a difference between the invertercurrent and the load current; determine a voltage of the outputcapacitor; generate a derived current based on the determined voltageand a predetermined capacitance of the output capacitor; compare themeasured current and the derived current to identify a fault location;and control the static switch based on the identified fault location. 8.The controller in accordance with claim 7, wherein said controller isfurther operable to: calculate a difference between the measured currentand the derived current; compare the difference between the measuredcurrent and the derived current to a predetermined threshold; anddetermine the fault location is at the associated UPS when thedifference between the measured current and the derived current exceedsthe predetermined threshold.
 9. The controller in accordance with claim7, wherein said controller is further operable to: calculate adifference between the measured current and the derived current; comparethe difference between the measured current and the derived current to apredetermined threshold; determine the fault location is at a locationexternal to the associated UPS when the difference between the measuredcurrent and the derived current is within the predetermined threshold;and control a circuit breaker of the UPS system to isolate thedetermined fault location.
 10. The controller in accordance with claim7, wherein the inverter current is generated by a first converter of aplurality of converters and the load current is an output current of theplurality of converters, wherein said controller is further operable to:receive, from the associated UPS, an alert signal indicating the firstconverter has failed to cause a UPS fault condition at the associatedUPS; and control the static switch to close in response to the alertsignal.
 11. The controller in accordance with claim 7, wherein saidcontroller is further operable to control the static switch to closewhen the determined fault location is at the associated UPS.
 12. Amethod for identifying a fault location within an uninterruptible powersupply (UPS) system including a plurality of UPSs and a ring bus thatelectrically couples the plurality of UPSs together, said methodcomprising: receiving, by a controller, current data representative ofan inverter current and a load current associated with a first UPS ofthe plurality of UPS s, wherein an output capacitor of the first UPS iscoupled to a node that conducts the inverter current and the loadcurrent; calculating a measured current based on the received currentdata, wherein the measured current is a difference between the invertercurrent and the load current; determining a voltage of the outputcapacitor; generating, by the controller, a derived current based on thedetermined voltage and a predetermined capacitance of the outputcapacitor; comparing the measured current and the derived current toidentify a fault location; and controlling, by the controller, a staticswitch associated with the first UPS based on the identified faultlocation.
 13. The method in accordance with claim 12, wherein comparingthe measured current and the derived current further comprises:calculating a difference between the measured current and the derivedcurrent; comparing the difference between the measured current and thederived current to a predetermined threshold; and determining the faultlocation is at the first UPS when the difference between the measuredcurrent and the derived current exceeds the predetermined threshold. 14.The method in accordance with claim 12 further comprising: calculating adifference between the measured current and the derived current;comparing the difference between the measured current and the derivedcurrent to a predetermined threshold; determining the fault location isat a location external to the first UPS when the difference between themeasured current and the derived current is within the predeterminedthreshold; and controlling a circuit breaker of the UPS system toisolate the determined fault location.
 15. The method in accordance withclaim 12, wherein the inverter current is generated by a first converterof a plurality of converters associated with the first UPS and the loadcurrent is an output current of the plurality of converters.
 16. Themethod in accordance with claim 15, wherein controlling the staticswitch further comprises: receiving, from the first UPS, an alert signalindicating the first converter has failed to cause a UPS fault conditionat the first UPS; and controlling, by the controller, the static switchto close in response to the alert signal.
 17. The method in accordancewith claim 12, wherein controlling the static switch further comprisescontrolling the static switch to close when the determined faultlocation is at the first UPS.